1. Field of the Invention
The present invention relates to power-on reset circuits. In particular but not exclusively the invention is concerned with power-on reset circuit arrangements for use in CMOS integrated circuits.
2. Description of Related Art
The purpose of a power-on reset circuit is to initialize logic and memory elements as the power supply voltage is raised from a very low level, that is, to set such elements to a known initial state. To do this the reset circuit must itself start in a known state. Reset circuits are used for example in calculators and electronic telephone sets.
The voltage at which a reset circuit is required to operate is a function of the operating voltage range of the associated circuits. If the reset circuit operates at too high a voltage, then logic and memory elements may be reset while still being supplied with a usable voltage, and in many applications the lower the voltage at which the reset circuit operates the longer the memory elements will retain stored information.
Reset circuits fall broadly within two categories, those in which the power supply voltage increases rapidly so that capacitors may be used to determine the reset state, and those in which the supply voltage increases or decreases slowly so that some form of level-detector circuit is required. Circuits designed for slowly changing supply voltages can usually be modified fairly readily, by the addition of capacitors, to deal also with rapidly changing supply voltages.
In some known reset circuits for dealing with slowly changing supply voltages the detection voltage level is determined by the sum of n and p channel transistor threshold voltage, plus an additional margin which may include the threshold voltage of a diode in series with the transistors. Such reset circuits are commonly referred to as sum-of-threshold circuits. However, memory elements will usually hold down to supply voltages of less than half of this sum-of-threshold voltage, so that such reset circuits tend to operate at too high a voltage.